1) Field of the Invention
The present invention relates to a memory control device that stores last memory data when power of a device is turned off, and reads the last memory data when the power is turned on.
2) Description of the Related Art
Some devices have a so called last memory function. A device is said to have the last memory function if, when power is turned off while the device is operating, data relating to the control settings of the device is stored in a recording medium just before the power is turned off. In such a device, when the power is turned on again, the control settings are read from the recording medium. As a result, the control settings of the device can be automatically restored to the one before the power is turned off.
Thus, the recording medium used in such devices must be able to retain data even after the power is turned off. Examples of such recording media are hard disk drives (HDD) and non-volatile memories. Examples of the non-volatile memories are flash memories, electrically erasable programmable read only memories (EEPROM), ferroelectric random access memories (FeRAM), and magnetoresistive random access memories (MRAM). The volatile recording media can also be used as the recording media. However, the volatile recording media require power supply from an auxiliary source to retain data. Examples of the volatile recording media are static random access memories (SRAM) and synchronous dynamic random access memories (SDRAM).
The flash memories are particularly advantageous; because, the flash memories are compact, light-weight, and data can be written on them in short time There are various conventional methods of writing data in the flash memory, such as those disclosed in Japanese Patent Application Publication No. H11-328982 and Japanese Patent Application Publication No. H8-272698.
The HDDs require large current, and takes time to write data because mechanical operations are required. On the other hand, with the flash memories, data writing cannot be immediately executed because data in the flash memory needs to be erased first. Erasing of data takes about 0.5 second to 2 seconds, and writing of data takes about 6 microseconds (μs) to 100 μs. Therefore, when the HDDs or the flash memories are used, the power needs to be retained for some time even after the power is turned off. This requires a high-capacity condenser, which occupies the space and increases the cost.
The EEPROM cannot store a large amount of data because it is small in capacity. The FeRAM and the MRAM are expensive because they are not yet mass produced. The SRAM and the SDRAM require auxiliary power, which not only leads to more space and cost, but also consumption of dark current.
There are devices that can be detached while the power is on, and the power is turned off when the device is detached. Such detachable devices are particularly required to quickly write data in a recording medium, so as to retain the data after being detached (i.e., after the power is turned off).